Well logging instruments have been used to measure earth formations along the traverse of a borehole for over 80 years. Most of the instruments designed for measuring the apparent formation bulk density of an earth formation have been derived from an instrument disclosed in U.S. Pat. No. 3,321,625 issued to Wahl in which a compensated gamma-gamma measurement is derived from measuring the mud-cake along the borehole wall using an eccentralized, collimated tool string. Well logging instruments are typically lowered into a well on electro-mechanical cable, solid wireline cable (slickline), conductor slickline, coiled tubing, drill pipe, tubing, continuous rod (COROD™) well tractor or via pump-down methods. The signals from the tool may be transmitted to surface through electro-mechanical cables with conductors (Surface Read Out or SRO), stored to a downhole tool memory (Memory Logging) or transmitted to surface via Mud-Pulse Telemetry or Electromagnetic Pulse Telemetry methods, such as those used in MWD (Measurement While Drilling) and LWD (Logging While Drilling) methods. The instruments lowered into the wellbore include sensors for measuring various properties of the earth formations.
Measurement of the apparent bulk density of an earth formation is particularly useful and has applications in determining the fractional volume of pore space in the earth formation that may contain oil or gas, determine the overburden force of an earth formation at any particular depth and to determine the mineral composition of an earth formation amongst other things.
Well logging instruments used for the determination of the apparent formation bulk density are well known in the prior art. Such instruments generally consist of a pad to contact the borehole wall and that is made of a high Z material to be impervious to radiation. The pad houses a steady-state isotopic chemical source, such as cesium-137, and a plurality of gamma-ray scintillation detectors that are spaced apart from the 137Cs source. The 137Cs source and the gamma-ray scintillation detectors are collimated toward the borehole wall and the tool is run decentralized so that the collimated windows are, preferably, in constant contact with the borehole wall. It is known to those skilled in the art that the collimated windows are made of a material that is transparent to gamma-ray radiation.
The use of two or more gamma-ray detectors spaced apart at different distances from the gamma-ray source is known to those skilled in the art as a method used to produce a formation bulk density log that is compensated for mud-cake effects, such effects being the mud-cake density and the mud-cake thickness. The Short-Space (SS) gamma-ray detector is primarily sensitive to gamma-rays that have been scattered in the mud-cake, while the Long-Space gamma-ray detector (LS) detector is primarily sensitive to gamma-rays that have been scattered in the formation. The outputs of the SS and LS gamma-ray detectors may then be used to generate a log of apparent formation bulk density that is corrected for mud-cake density and mud-cake thickness through the use of a “Spine and Ribs” correction, known to those skilled in the art. Essentially, if a volume of low-density mud is between the pad and the formation to be measured, and if the mud density is less than the formation density, then a positive density correction is applied to the LS density reading to correct for this condition. Alternatively, if the mud density is greater than the formation density, then a negative density correction is applied to the LS density reading to correct for this condition. This correction is only valid if the tool standoff is minimal and the density pad is in direct contact with the formation. It has been disclosed in prior art that for thin mudcakes up to 0.5 in., the spine and ribs correction for mudcake thickness is insensitive to the mudcake density. When the mudcake exceeds 0.5 in., the mudcake density introduces significant errors into the corrections.
This problem in the prior art detectors is attributed to data taken from two detectors which does not provide any means of taking into account differences in mudcake density in correcting the apparent density from the far detector count-rate. If the borehole wall is rugose and gaps appear between the density tool pad and the borehole wall, the apparent formation bulk density may not be determined due to the excess mud volume, the mud density and the mud thickness between the pad and the earth formation, a condition often referred to as excessive standoff. In a cased-hole application, a built-in standoff is created by the casing wall and the cement thickness, mitigating the pad from directly contacting the formation.
The use of a high Z material to shield the gamma-ray source and the gamma-ray photon detectors serves to collimate the gamma radiation into a beam and shield the gamma-ray detectors from incident gamma-ray radiation that is outside of the scattering or collimation region. The collimated gamma-ray emissions from the gamma-ray source enter a region of the earth formation where the gamma-rays can interact or collide with the electrons orbiting the atoms in the materials comprising the earth formation. Each collision can cause a gamma-ray to lose some of its energy and be deflected from its original path or direction of travel and is known to those familiar with the art as Compton Scattering. Photoelectric Absorption occurs when the gamma ray energy is so low that the gamma-rays are absorbed by electrons. A 137Cs source is typically preferred as the gamma-ray energy of 137Cs (662 keV) falls within the region of Compton Scattering events and with a half-life of 30.23 years, offers a stable source of gamma-ray emissions. Collimation of the gamma-ray source and the gamma-ray detectors only allows for the detection of backscattered gamma-ray radiation through a certain range of angles, thus only a certain volume can be detected, essentially providing a one-dimensional response.
Conventional pad-type formation bulk density tools have been run in steel-cased wellbores in attempts to derive a formation bulk density and locate gas, oil and water horizons. The success of these tools has been very limited in providing a quantitative measure of the apparent formation bulk density of an earth formation primarily as a function of the tool count-rates being severely affected by the collimation of the gamma-ray source and scintillation detector(s), the steel casing, the cement or annular density of the fill material behind the casing and the unknown thickness of the cement or annular fill material behind the casing.
Further, the collimation of the tools precludes any form of spectral de-convolution being used to resolve the apparent formation bulk density using Compton ratio methods. Still further to this, the tool collimation results in a one-dimensional measurement that effectively eliminates a large fraction of the wellbore region from being measured. Collimation also has a further effect of reducing the depth of investigation of the tools resulting in shallow measurements that typically cannot extend beyond the near wellbore region of interest in order to correct for the environmental effects of the annular fill region. Subsequently, a compensated apparent formation bulk density measurement may not be achieved under the vast majority of wellbore conditions as a direct result of the shallow depth of investigation of collimated instruments and an inability to quantify and correct for these effects. Additionally, casing eccentricity in the borehole creates a non-uniform annulus that may have varying degrees of annular-fill thickness surrounding the periphery of the casing at any given point. These additional stimuli all affect the true formation bulk density response and require to be resolved in order to correct the measured tool responses for these perturbations to produce a correction compensation factor that accounts for all of the aforesaid unknowns. Prior art has still relied upon a conventional “Spine and Ribs” approach correction that has limited application to the problem and may only be applicable under ideal conditions wherein the combined casing thickness and cement thickness fall below a known value, typically 1.5 inches. Of the numerous factors related to “correcting” the apparent formation bulk density measurement in a through-casing environment, the casing has been treated as a constant factor based on the rate of gamma-ray attenuation through various thicknesses of steel casing and is not measured directly, thus, variations in casing thickness are not measured or corrected for.
In U.S. Pat. No. 4,129,777 issued to Wahl et al, a method is disclosed of measuring cement thickness using a collimated density tool and using the thickness value as an index on the reliability of the formation density measurement. As disclosed by Wahl et al, as long as the cement thickness does not exceed one inch, the density reading from the far-space count-rate can be considered as reflecting the true porosity of the formation. In the same patent, Wahl et al, also discloses that if the cement thickness exceeds one inch, then the far space count-rate measure of formation porosity has not been accurately compensated for the effect of the cement layer. It is easy to see that this method relies upon the optimal condition of having a thin, uniform cement annulus and no pipe eccentralization is prevalent. Conditions such as this are seldom seen in reality.
In U.S. Pat. No. 5,525,797 issued to Moake, a three detector, multi-collimated tool is described that is reportedly effective in either cased-hole or open-hole although no tool was ever built to confirm the claims. In the Moake patent, the gamma-ray source is collimated and each of the three detectors retains a different angle of collimation to provide multiple depths of investigation to measure the annular effects behind casing. The Moake tool, unlike conventional art density tools, is not a pad-type device but does require to be decentralized against the borehole or casing wall. The Moake patent also refers to the use of multiple detector spacing ratios to match the anticipated cement thickness expected in the well. Essentially this requires that the source-to-detector spacing of each scintillation detector requires to be adjusted to increase or decrease the depth of investigation based upon the reported well conditions which are unknown and may only be inferred from assuming ideal conditions. Such a tool configuration demands that either multiple tools be made available with various source-to-detector spacing's or either an adjustable detector chassis complete with numerous collimated pressure housings to match the scintillation detector windows be made available as a matter of utility. Either method precludes any real practicality of the device. Further to this, like the prior art patents previously mentioned, collimation creates a one-dimensional measurement that eliminates a large fraction of the well bore region from being measured, thus the tool azimuthal bearing in the well will render different responses in the same well from run to run as the tool rotates.
In U.S. Pat. No. 5,390,115 issued to Case et al, another collimated tool, uses a collimated gamma-ray source and three collimated detectors. Case professes to have a method in which to estimate the mudcake thickness independent of the mudcake density. The Case patent was focused solely on open-hole logging and it wasn't until 12 years later that U.S. Pat. No. 7,292,942 B2 to Ellis et al was published that employs the same tool technology but with a claim to measure formation density through casing. The Ellis patent essentially exploits the technology of the Case patent with the claim that the Ellis patent is only capable of determining the apparent formation bulk density through-casing when the maximum combined thickness of the casing and cement is less than 1.75 inches or 1.85 inches. Should these conditions be exceeded, then the well log data is flagged as having too great a stand-off to be accurately measured.
As with the previous prior art patents, the Ellis patent is based on a collimated gamma-ray source and collimated scintillation gamma-ray detectors with a limited depth of investigation, a narrow one-dimensional borehole volume and self-described limitations in a cased-hole.
The primary reason for the inability to correct for these factors using current state-of-the-art collimated technology is due to the scattering geometry principles in use being directly affected by these conditions and through an increased tool stand-off whereby the density pad is not in direct contact with the formation wall. These effects all contribute to variations in the LS and SS count-rates to the point that a conventional “Spine and Ribs” approach correction may not be applied to the individual count-rates as the effects of the near wellbore region may not be properly compensated for or adequately measured. This includes the casing thickness, the cement or annular mud thickness, the cement or annular mud density and the tool stand-off itself as a result of borehole rugosity. Patents relating to the use of an eccentered, collimated pad-type density tool run in steel casing have also been granted to Reeves Wireline Technologies. (Ref. Patent U.S. Pat. No. 6,909,969, Jun. 21, 2005 and Ref. Patent U.S. Pat. No. 7,328,106, Feb. 5, 2008). The Reeves '969 Patent claims a ‘Method of Through-Casing Gas Detection’, while the Reeves '106 Patent claims a ‘Method of Correcting Density Logs for the Presence of Casing’. In both the '106 Patent and the '969 Patent, Calvert et al claims to treat the casing as a high-density mudcake in order to apply a conventional “Spine and Ribs”, Short-spaced density versus long-spaced density correction, however, the method is incapable of correcting for the irregular cement-filled annulus encircling the steel casing. Furthermore, the dimensions of the annulus are such that they affect the accuracy of the “Spine and Ribs” method, therefore precluding the use of this technique as a means to quantify bulk density through-casing. This method has been documented by Elkington et al (Weatherford, 2006) to be ineffective when the combined casing wall and cement thickness approach a thickness of 1.35 inches and further claims that all sensitivity to formation density is lost when the stand-off exceeds 1.5 inches.
The presence of casing is but one factor that must be overcome to properly measure the annular space between the steel casing and the earth formation in order to derive an apparent formation bulk density.
With the current state-of-the-art collimated pad type devices, the pad must be pressed directly into the borehole wall to provide direct contact of the pad through the mud-cake to measure the apparent formation bulk density. Similarly, a collimated, “slick” (non-pad) borehole sonde may run in casing or open-hole to provide a qualitative measure of bulk density by decentralizing the sonde against the casing wall or open-hole. Such “slick” (non-pad) collimated devices have inherited the same deficiencies as the pad-type devices as a direct result of using the same tool geometry. Furthermore, for a collimated pad-type skid to be effective in a cased-hole and in order to preclude any wellbore fluid becoming interposed between the density pad and the casing, the pad radius must be designed to tightly follow the internal casing diameter radius to mitigate any fluid being “trapped” between the pad and the casing wall which would have the effect of introducing more error into the density measurement. This would require multiple pad curvatures to match the numerous casing dimensions used in the industry in order to remove this effect.
Contained within the density pad is a source of gamma-ray radiation oriented to the borehole wall through a collimator and two gamma-ray detectors that are also collimated to the borehole wall. Similarly, in the non-contact “slick” (non-pad) sonde, the source of gamma-ray radiation and the gamma-ray detectors are also collimated. This tool geometry is described as “single-scatter” geometry.
In both collimated tool configurations, in an open-hole environment, the collimation of the gamma-ray source and the gamma-ray detectors works to reduce the radiation source intensity bombarding the formation; reduce the depth of investigation; reduce the gamma-ray detector count-rate intensity; reduce the earth formation sampling volume as only those gamma-rays scattered through a certain range of angles will be detected; reduce the borehole sampling region; reduce the count-rate (detector) statistics; reduce the count-rate (detector) sensitivity; increase the radiation hazard to personnel; create poor repeatability; create a small, focused measurement volume; respond to both Compton scattered and photoelectric absorption gamma-rays; limit the amount of mud cake thickness correction that may be applied as a result of a reduced depth of investigation due to collimation (typically ¾″); render a spine and ribs correction method to correct for mud cake thickness ineffective for mud cake thickness greater than ¾″; produce an invalid measure of the formation bulk density when mud cake thickness exceeds ¾″ or in regions of rugose hole conditions and, which is further influenced in a cased-hole by; a further reduction in count-rates as a direct result of gamma-ray attenuation through steel casing; a further reduction in count-rates as a direct result of gamma-ray attenuation through cement as a function of the cement density; a further reduction in count-rates as a direct result of gamma-ray attenuation through cement as a function of the cement thickness; an inability to correct for the cement-filled annulus as a result of the cement density and cement thickness (i.e. “spine and ribs” industry correction); an inability to account for or correct for hole geometry (casing eccentricity, hole ovality, hole rugosity, etc); an inability to measure the cement density as a result of too few gamma-ray detectors; an inability to measure the cement thickness as a result of too few gamma-ray detectors; an inability to measure the casing thickness as a result of too few gamma-ray detectors; an inability to compensate for penetration by increasing the source-to-detector spacing as a result of further decreasing the count-rates reaching the detectors; a further reduction in depth of investigation as a direct result of above cased-hole conditions; an inability to measure any photoelectric absorption gamma-rays; an inability to derive a delta rho correction, therefore; an inability to derive a compensated formation bulk density value.
Additionally, the application of gamma-ray spectroscopy would be ineffective in a collimated system as a direct result of too few count-rates leading to a lack of sensitivity for each variable to be analyzed.
In a US Patent Application, US 2010/0252725 A1, to Stewart, published Oct. 7, 2010 and now abandoned, a method was described for determining the bulk density of an earth formation using an omni-directional radiation source and a pair of non-shielded or non-collimated radiation detectors. This configuration was run centralized in a wellbore as opposed to eccentered and offered a new approach to measure the bulk density of an earth formation through steel casing or in an open-hole.
As set forth in the Stewart application, a method for obtaining information indicative of the bulk density of a subterranean earth formation penetrated by a wellbore was provided comprising irradiating the formation with a non-collimated emission of gamma ray radiation produced by a gamma ray source; and establishing measurements, such as count-rates, of non-collimated scattered gamma rays produced by the formation in response to such irradiation and detected by a pair of gamma ray detectors. A non-collimated gamma ray source was operative to irradiate a subterranean formation with a non-collimated emission of gamma ray radiation; and a pair of non-collimated gamma ray detectors spaced axially from the source, each operative to detect and measure a count-rate of non-collimated scattered gamma rays produced by the formation in response to such irradiation. Optionally, the tool is centralized in the wellbore in the course of logging.
In Stewart and in for purposes of this specification, the following terms have the following meanings: “substantially non-shielded” means that each of the source and detectors are shielded only to the extent needed to effectively prevent radiation moving up or down axially along the tool to deleteriously affect the working of the detectors, but are otherwise generally non-shielded on the sides so as to respectively emit or collect gamma rays in a non-collimated manner, substantially omni-directional and “generally spherical” in effect.
Regardless of Stewart's move to a non-collimated source, and the Stewart system was capable of reproducing an approximate formation bulk density measurement in most situations, it was found that dynamic human intervention was required to “fine-tune” the responses. The linear transforms methodologies were vulnerable to variations in any one of the variables of density and thickness. Accordingly, it appears as through the Stewart system, as disclosed, is no able to derive a borehole compensated bulk density if run in a “blind” test. Overall, Applicant believes it would be impossible for the Stewart tool described in US 2010/0252725, in its current configuration, using gross gamma-ray count-rates, to derive all of the necessary measurements to correct for borehole conditions to acquire an apparent bulk density measurement.
Further, as with the Reeves 106 B2 and 969 B2 Patents, irregular annulus dimensions would most likely preclude the use of an industry standard “Spine and Ribs” correction. Neither Reeves patent is able to provide a measure of: an apparent formation bulk density of an earth formation; an apparent cement or annular density measurement; an apparent cement or annular thickness measurement; a casing thickness measurement; a measure of the tool stand-off to the formation; or a measure of the borehole fluid density.
Given the realities of well logging, Applicant believes this would be an impossible situation to predict or overcome in a real-life application due to a number of factors, some of which are identified as borehole dimensions vary due to lithology changes, mud type, tectonic stresses. Casing is seldom centered in the borehole as this would require the use of multiple centralizers to be ran over the depth of the entire well. Wells are often drilled at some angle from vertical, albeit minor, introducing more casing eccentricity Poor mud removal during primary cementing may alter the cement thickness across any given depth interval. Poor mud removal during primary cementing may preclude any cement being placed across a given interval. Cement hydration during curing may shrink the cement sheath away from the casing or formation or both. Cement may become invaded with gas and or water as it cures, effectively reducing the cement density and cement volume behind the casing. A delta RHO or density correction, might not be derived as a result of variations in annular thickness and the limitations of the tool depth of investigation relative to the detector spacing's used. An apparent formation bulk density could not be derived as a result of the failure of the correction and further, if the detectors were collimated, the collimation angles would also need to be varied for each detector in order to alter the depth of investigation required as a result of varying thicknesses.
Clearly there is still a need for well logging apparatus which overcomes the problems associated with the prior art devices.